Measurement of the biomass is very important for monitoring and operating bioreactors, both in process development laboratories and in production facilities.
Usually this measurement is performed either by removing a sample of the reaction medium then counting the number of cells under the microscope in the laboratory, which is a very long process, i.e., more than 12 hours, or by calculating the optical density, which is the logarithm of the ratio between the "entering" intensity of a light flux upon its emission and the "exiting" intensity of this same flux after it has passed through a certain length of a sample of the reaction medium contained in the fermenter. For a given reaction, the optical density in transmission is a value that is well correlated with the quantity of biomass of the reaction medium, and the measurement thereof permits indirectly evaluating the bacterial concentration of the medium.
In a device for measuring optical density manufactured and sold by the Swiss company CHEMAP, the measurement is performed in a cell situated outside the fermenter. This measuring device is connected to the fermenter by a thin tube with a length of approximately several tens of centimeters, through which there is sucked the sample of reaction medium upon which the measurement is performed, the tube remaining permanently open to the fermenter. The operating principle of the cell is quite similar to that of a syringe, the measuring cell belonging to the reservoir of the syringe: a portion of the liquid contained in the fermenter is first sucked into the cell and, to avoid the situation that the bubbles contained in this liquid falsify the measurement of optical density, the liquid is allowed to rest for some tens of seconds, the time in which the bubbles migrate to the upper part of the cell. The measurement is then performed by comparing the intensity of the light flux at the entrance of the cell with the intensity of this flux at the exit of the cell, then, once the measurement has been performed, the liquid is reinjected into the fermenter. To take into account possible variations of the intensity of the light source emitting the light flux, the optical density through the liquid sample and the light intensity emitted by the source are measured simultaneously. Thus the result can be calibrated. In addition, a system of small wipers permits, in this device, cleaning by scraping the optical surfaces between two measurements.
A first drawback of this apparatus is its bulkiness; a second drawback is that there exists a risk of defective sterilization and, because the tube is open to the medium, of pollution of the fermentation process. Routine use of this apparatus can pose problems in production.
In U.S. Pat. No. 4,725,148, there is described a device that permits measurement in situ, therefore without sampling steps, of the optical density of the reaction medium contained in a fermenter. The device in this case is inside a mechanical system sunk into the shell of the fermenter, such that its lower part dips into the reaction medium.
In the lower part of the apparatus there is provided, by way of a cell, a space that is open to the reaction medium such that this circulates freely therein. The measurement of biomass in this case is also performed by comparison of the intensity of an emitted light flux and the intensity of this same flux received after passage through a known thickness of the reaction medium to be studied. The light source of this device is a semiconductor laser diode providing a constant intensity, such that calibration is not necessary, and the receiver is a semiconductor diode. A cooling loop limits the temperature of the device, such that the apparatus can remain in place during the sterilization phases. To avoid the situation that the bubbles disturb the measurement, a screen is placed at the entrance of the cell. This screen must not be too fine, in order that it not prevent the large particles contained in the reaction medium from entering the cell: the measurement performed would then not be representative of the real state of the medium, especially at the end of fermentation. The small bubbles of diameter smaller than the spacing of the screen will therefore be able to perturb the measurement: if, for example, the rate of agitation of the medium is changed, the number and the size of the bubbles vary and the measurement risks being different. Another problem is that this screen becomes fouled quite rapidly and that it is not provided with means for cleaning it.